By way of summary, in the dental field of application which has been taken as an example, the treatment of dental caries in particular by the direct method is frequently performed by the use of a compound resin or of an amalgam which the dental practitioner deposits within the previously cleaned cavity, with solidification of the amalgam being achieved by setting and/or by means of the microretentions situated upon the walls of the cavity.
In the case of an anesthetic compound resin, hardening is achieved by means of polymerization with the addition of a chemical substance, or by means of electromagnetic radiation.
With regard to the latter case, the process currently in use involves, after first having cleaned the walls of the dental cavity to be treated, depositing in a number of successive layers a mass of photosensitive compound resin intended to fill the cavity, and then carrying out hardening, layer by layer, by means of photopolymerization of the resin as a result of the use of electromagnetic radiation such as, for example, luminous radiation emitted by means of a light source, the wavelength of which corresponds to the reaction for the initiation and solidification of the resin.
This photopolymerization process offers appreciable advantages in relation to the customary chemical techniques which are still being practiced at this time, since it no longer requires rapid intervention on the part of the dental practitioner as a consequence of the short setting time of the hardener.
These disadvantages characteristic of the customary chemical techniques also do not exist in the method which uses a photosensitive resin, since the photopolymerization reaction of the viscous resin does not commence until the moment the same is activated by means of the radiation triggered by means of the dental practitioner.
It has been found, however, that certain patients treated by means of this method experienced acute pain because the resin adheres to the walls of the tooth, in particular to the walls of the pulpar chamber where the nerves and the blood vessels are situated, placing them under strain as the resin shrinks. In addition the polymerization, which is always incomplete at the base of the cavity, leaves behind free monomers at that point which are toxic to the dental pulp.
In the case of industrial applications, for example in the field of precision molding, this shrinkage phenomenon, which is well known to the professionals, represents a distinct disadvantage in that it is detrimental to the quality and the precision of the molded and the moulded surfaces.